The present invention relates to semi-solid molding (SSM) of metal alloys and the equipment and methods used for SSM, and which are disclosed in many U.S. and foreign patents, for example, in U.S. Pat. No. 3,954,455, No. 4,434,837, No. 5,161,601 and No. 6,165,411. SSM is also discussed in technical publications, for example, in a book entitled Science and Technology of Semi-Solid Metal Processing, published by North American Die Casting Association in October, 2001. Chapter 4 of this publication was authored by a co-inventor of the present invention. In conventional SSM processes, it is necessary to use either a specially treated, pre-cast billet of appropriate microstructure or a slurry especially prepared from molten alloy in equipment external to a die casting press. The cost premiums associated with either the pre cast specially treated billet that must be sawed to length before using, or the slurry especially prepared in equipment external to the die casting press, have severely limited the commercial applications of the SSM processes. Also, the pre-cast billet is available from a relatively few sources, is currently made only from primary alloys, and process offal cannot be reused unless reprocessed back into a billet.
Still, SSM provides some important and highly desirable characteristics. Unlike conventional die castings, die cast parts which are produced using SSM processes can be produced substantially free of porosity, they are able to undergo high temperature thermal processing without blistering, they can be made from premium alloys, and they provide reliable high levels of strength and ductility when made using appropriate alloys and heat treatments. Because of the thixotropic nature of semi-solid slurry and the non-turbulent way that relatively viscous thixotropic slurries flow in die casting dies, the SSM process is capable of producing cast parts having thin sections, great detail and complexity and close dimensional tolerances, without the entrapped porosity and oxides which are commonplace in conventional die casting processes.
The present invention is directed to a new SSM process or method which significantly reduces the costs of producing parts by the SSM process. The method of the invention is ideally suited for producing parts having thin sections, fine detail and complexity and close dimensional tolerances, and which are substantially free of porosity and oxides, can be processed at elevated temperatures without blistering and which can provide high and reliable levels of strength and ductility. The method of the invention avoids any need to produce a specially treated, pre-cast billet that must be sawed to length before using or a slurry especially prepared from molten alloy in equipment external to the die casting press. The method of the invention is also applicable to a wide variety of alloys, for example, standard A356 alloy and alloys of the Alxe2x80x94Si, Alxe2x80x94Cu, Alxe2x80x94Mg and Alxe2x80x94Zn families, all of which can be acquired in the form of and at prices normal to conventional foundry ingot, including both primary and secondary origin.
In accordance with one embodiment of the present invention, an ingot of commercially available solid metal or metal alloy, such as aluminum foundry alloy ingot, is heated to the molten state. If not permanently grain refined, such as by employing a foundry alloy called SiBloy produced by Elkem Aluminum, AS, an xcex1 aluminum grain refining material such as 5:1::Ti:B master alloy produced by numerous suppliers, or a product called TiBloy produced by Metallurg, is added to the molten alloy in appropriate quantities to accomplish fine grains in the solidified alloy product. The grain refined molten alloy is poured directly into a large diameter shot sleeve or chamber of a vertical die casting machine or press. The shot chamber receives a vertically movable shot piston which forms the bottom of the shot chamber, and the diameter of the shot chamber is greater than its depth or axial length. In a preferred embodiment of the present invention, the shot chamber is greater than its depth by a ratios of 2:1 or more. The shot chamber is then indexed from the initial filling position to a slurry injection position under a die. The molten alloy is permitted to cool within the shot chamber to a predetermined temperature range in which it forms a semi-solid slurry having 40 to 60 percent solid, the solid fraction having a globular, generally non-dendritic microstructure. The portion of the slurry immediately adjacent to the wall of the shot chamber or shot sleeve and the shot piston become significantly colder and more solid.
When the semi-solid slurry within the central portion of a first shot chamber, now in the slurry injection position under the die, has cooled to the predetermined temperature range in which it has 40 to 60 percent solid, the shot piston is moved upwardly by a mechanical actuator or a hydraulic shot cylinder to transfer or inject the semi-solid slurry within the central portion of the shot chamber through one or more gate or sprue openings and into one or more cavities in the die above the shot chamber. The more solid portion of the slurry adjacent the shot sleeve is prevented from entering the die cavity or cavities, either by appropriately distancing the gate or sprue openings from the shot sleeve walls or by entrapping the more solid portion within an annular recess in the gate plate through which the gates or sprue openings communicate with the die cavity or cavities. As a result, the more solid portion of the slurry remains in the residual solidified biscuit. After the semi-solid slurry solidifies in the die cavity or cavities, the shot piston retracts to retract the biscuit intact with gates or sprues. The shot chamber is then transferred or indexed back to its initial filling position where the biscuit with the gates is removed laterally from the shot chamber and piston, and the shot chamber is then ready to repeat the cycle. After the die is opened, the part(s) is ejected and then indexed to a position where it is removed, and the die is ready to repeat the cycle.
During the slurry forming, slurry injection and slurry solidification steps described above relative to the first shot chamber while in its shot position, a second shot chamber in the original filling position has similarly been filled with grain refined molten alloy. When the first shot chamber and its piston are transferred or indexed back to the initial filling position for biscuit removal, the second shot chamber and molten alloy are indexed to the metal transfer or slurry injection position under the die, and the process of slurry formation, slurry injection and slurry solidification is accomplished just as with the first shot chamber. The process is repeated over and over again.
Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.